Pressure-sensitive adhesive tapes are well known in the art. An "adhesive", as defined by G. G. Hawley in "The Condensed Chemical Dictionary", 9th Edition, Van Nostrand Reinhold: New York (1977), is "any substance, inorganic or organic, natural or synthetic, that is capable of bonding other substances together by surface attachment". A pressure-sensitive adhesive (PSA) is generally a component of a pressure-sensitive tape which in its most simple configuration is comprised of the adhesive and a backing, and the overall construction is tacky and adherent at the use temperature (typically room temperature) and adheres to a variety of substrates using only moderate (typically fingertip) pressure to form the bond. In this fashion, pressure-sensitive tapes constitute a complete, self-contained bonding system.
A good pressure-sensitive adhesive, according to D. W. Aubrey in "Developments in Adhesives", ed. W. C. Wake, Vol. 1, Chapter 5, Applied Science Publishers, London (1977), must fulfill three main technical requirements:
1. The adhesive must exhibit adequate resistance to shear under the influence of an applied load; this relates to the adhesive's cohesion.
2. The adhesive must conform quickly to the surface to which it is applied in order to rapidly form a bond; this relates to tack.
3. The adhesive must display adequate resistance to separation by peeling once the bond is formed; this requirement relates to adhesion. These three requirements are assessed generally by means of tests which are designed to measure cohesion (shear holding power), tack, and adhesion (peel strength). These measurements taken together constitute the "balance of properties" often used to characterize PSA's.
Of the parameters that have relevance for a PSA in the bonded condition, namely cohesion and adhesion, numerous methods have been developed for increasing cohesion of PSA's subsequent to preparation and coating of the adhesive, e.g., various crosslinking reactions. Adhesion, on the other hand, can as a rule be controlled only at the stage of adhesive preparation. Adhesion is influenced by bulk viscoelastic behavior (related to the polymer glass transition temperature, Tg) and interfacial attractive forces between the adhesive and substrate. These factors are regulated by the choice and ratio of the starting monomers. Thus, beyond the typical, modest build in adhesion observed fairly soon after bonding which results from flow or creep of the polymeric adhesive into a condition of more complete contact with the substrate surface, no effective measures have been developed to significantly increase adhesion in the bonded condition.
Copolymers derived from alkenyl azlactone and acrylic-functional carboxylic acid monomers have been prepared and disclosed in British Pat. No. 1,121,418 and U.S. Pat. No. 3,583,950. Both of these patents incorporate an alkenyl azlactone monomer in a copolymer expressly for the purpose of ultimately crosslinking the copolymer. These patents broadly teach the use of the crosslinkable, solid azlactone copolymers derived from at least three monomers in admixture with a suitable crosslinking agent (e.g., a diol or diamine) as "coating compositions". A limited expansion upon this proposed utility is found in column 5, lines 16-20, of U.S. Pat. No. 3,583,950, wherein it is stated that "the mixtures of the invention have a potlife sufficient to use them as coating or impregnating agents, as adhesives, or--optionally together with fillers--as patching materials." No mention is made of the possibility of utilizing azlactone-containing copolymers themselves as adhesives.
Normally tacky, pressure-sensitive acrylate copolymer adhesives are disclosed in U.S. Pat. No. Re. 24,906.